FIG. 1 illustrates a prior art variable gain amplifier (VGA) based on a continuously interpolated attenuator. The circuit of FIG. 1 includes an attenuator network 100, a series of transconductance (gm) stages 102, an interpolator 104, a main amplifier 106, and associated support circuitry. The attenuator receives an input signal VIN and generates a series of progressively attenuated signals at a row of output tap points. Each gm stage is coupled to one of the tap points to receive one of the attenuated signals. The outputs from the gm stages are connected together and provided to the main amplifier so that the overall output signal is the sum of the output signals from all of the gm stages.
The interpolator steers a bias current IE to the gm stages as a series of interpolation signals I1 . . . I8 in response to a control signal VCTRL. When the highest gain is selected, all of the bias current IE is steered to the gm stage closest to the input end of the attenuator. Therefore, the first gm stage is active, and the remaining gm stages are effectively off. As the gain is reduced, the interpolator steers the bias current to gm stages further away from the input end of the attenuator, thereby selecting gm stages that receive progressively attenuated versions of the input signal. The gm stages are sequentially enabled and disabled in a continuous manner in which one of the interpolation signals gradually increases while the adjacent interpolation signal gradually decreases.
The VGA illustrated in FIG. 1 is generally intended to cope with varying input signals. That is, the domain of application is typically those situations where the input may have a very wide range of amplitudes, and where the system requires an output that is normalized to some constant value, which might be, for example, the full-scale capacity of an analog-to-digital converter. In the specialized field of variable gain amplifier design, such a structure has been called an “IVGA”, meaning a VGA whose function addresses the wide range of signal amplitudes present at the input of the element. On the other hand, a structure that is expressly designed to accept an essentially constant input amplitude while providing an output signal of widely varying amplitude is called an “OVGA”.
Some examples of interpolators used in variable gain amplifiers having interpolated attenuators are disclosed in U.S. Pat. Nos. 5,684,431 and 5,077,541, both having a common inventor with the present application. Another example of an interpolator is disclosed in U.S. Pat. No. 5,432,478 also having a common inventor with the present application.